Answered By: Writing Center Last Updated: Apr 20, 2015 Views: 2
Must is used following the format: subject + must + infinitive. It is used for actions that need to happen, for example: We must go help her. She needs us. It is also used for things one believes are definite, such as: You haven't slept yet. You must be really sleepy.
Can't is used following the format: subject + can't + infinitive. It is used for actions that are not possible, for instance: You can't be that scared. That's just a movie! It's not real.
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